Optical amplitude modulation system capable of cancelling high order component

Abstract

It is an object of the present invention to provide a DSB-SC system capable of suppressing a third order component. The DSB-SC modulation having high extinction ratio can be realized by adjusting the first order component, which is generated by applying a modulation signal (3fm), and the third order component, which is generated by applying a basic signal (fm), to have reversed phase and the same intensity level, and then by applying the first order component to the third order component, these two components cancel each other.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention basically relates to an optical amplitude modulation system. In particular, the present invention is directed to an optical amplitude modulation system which is capable of performing high extinction ratio modulation by suppressing high order components which are generated when DSB modulation is performed.[0003]2. Description of the Related Art[0004]In optical communication, light is modulated to have signal information. As optical modulation, direct modulation and external modulation are known. The direct modulation modulates a driving power of semiconductor laser. And the external modulation modulates light from semiconductor laser by means other than light source. A modulator used in direct modulation is generally called an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on signals. As technical problems ...

AI Technical Summary

Benefits of technology

[0023]A phase modulation and / or an intensity modulation are performed to the output light from the optical amplitude modulator, or a modulation is performed to an input light so that the unsuppressed components are suppressed. In this process, phase and timing of sidebands offset form the first sidebands (whose frequency is equal to that of a carrier or a high order component) are adjusted so that carrier components (or high order components) are suppressed. Thus, carrier components (or high order components) can be suppressed, thereby realizing an optical modulation system which can perform high extinction ratio modulation.

[0030]This embodiment of the optical amplitude modulation system is preferable because the fiber grating and the circulator removes unnecessary components and the removed components can be used as feedback signals.

[0034](iii) decreases bias voltage of either one of the sub Mach-Zehnder electrodes so as to decrease output from the Mach-Zehnder waveguide (8), and

[0041]By using the bias voltage having been adjusted in this way, in other words, by using a signal with relatively less carrier components or relatively less high order components, these components are suppressed by the method above explained. Therefore, the components to be suppressed can be suppressed more effectively.

[0043]This system, the same as the optical amplitude modulation system according to the fifth aspect of the present invention, effectively derives a predetermined optical signal, then isolates unnecessary optical signal components and controls a signal source system so that the intensity of the isolated components to be lowered. Thus, unnecessary components are effectively reduced.

[0045]The present invention is able to provide an optical amplitude modulation system which is able to suppress a carrier component (f0) and a high order component (e.g. a second order component (f0±2fm)) and obtain high extinction ratio.

Problems solved by technology

Since the DSB-SC modulator and the optical SSB modulator have different voltage levels applied and different methods of applying the voltage, there is a problem that a method of removing the third order components which is taken by the SSB modulator cannot be applied to the DSB-SC modulator.

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